In a spread spectrum CDMA system, multiple communication units, or mobile systems, operate on a single frequency separated only by a particular CDMA code and/or a CDMA code delay defining a CDMA channel. Typically these CDMA channels are available throughout a cell, or sector of the cell. However, radiation of a CDMA channel within an area of the cell in which the particular communication unit assigned to the CDMA channel is not operating only serves to increase the interference energy experienced by other communication units operating on that frequency.
In order to obtain performance from a multi-beam antenna with a CDMA base station, the system must be able to despread and separate the transmitted and received signals that are using different codes or different code delays. In this way, it would be possible to restrict radiation of a particular CDMA channel to only within an area most likely to actually be utilized by a communication unit assigned to the CDMA channel. However, in order to isolate transmission of a CDMA channel to a particular area within the cell, a multi-beam antenna system is preferably utilized.
Accordingly, there exists a need in the art for a system and method for equipping a CDMA base station to dynamically restrict communication with communication units to selected beams of a multi-beam antenna system. However, a problem exists in the use of such a multi-beam antenna with a typical CDMA base station in that the traditional inputs and outputs from such a base station are at the radio frequency level. They contain a composite of all of the transmitted and received signals on the same frequency. It is difficult, if not impossible, to separate one user signal, or channel from another unless the system obtains information about the transmitted and received codes and the timing of those codes so that the traffic channel signals for one user can be broken apart from the signals of another user.
In the literature on CDMA systems, there is work that has been published and examined for integrating or combining multi-beam antennas with CDMA systems on the reverse link because it is relatively easy to model and relatively easy to get an estimate of the interference environment and the channels on the reverse path. It shall be appreciated that, unlike the reverse link, there are significant problems in modeling the forward link of a CDMA system. Likewise, getting estimates of the interference environment and the channel conditions of the forward link of a CDMA system is very difficult. Accordingly, a need exists in the art for a scheme of simplifying the processing on the forward link while still getting an interference reduction and a capacity gain on the forward link.
A need in the art exists for a system and method adapted to continuously switch the best signals from a multi-beam antenna system into the demodulation receiver of a CDMA base station in order that the receiver is able to make best use of all of the signals that are available from this multi-beam antenna.
A further need exists in the art for a system and method adapted to update selection of the best signals from a multi-beam antenna system in such a way that the channel environment and the motion of the mobile does not change too dramatically over the time in which the switching is accomplished.
A need also exists in the art for a system and method adapted in the forward link to transmit only on a minimum number of beams that have a good communications link to the mobile. Accordingly, transmit energy is not wasted on beams that are not radiating in the direction of the mobile and, thus reduction of overall interference on the forward link and improved capacity of the forward link is experienced.